For electric cables to form reliable links, such as are required for the aeronautical industry, they must be of top quality. The three most important qualities include: flexibility of the conductor, so as to avoid the danger of breakage This quality is obtained by using a stranded conductor constituted by individual copper strands each with a diameter of a few tenths of a millimeter. The second is electrical and mechanical behaviour of the insulation which is necessary to prevent short circuits and is provided by efficient insulating materials. The third is reliability of the connections formed at the ends of these cables by soldering or crimping. Such reliability is obtained by metal coatings which are deposited on the strands to prevent surface oxidation of the copper and to facilitate electric contact.
Polyimides such as, for example, those sold by the American firm Dupont de Nemours in the form of tape under the trade marks Kapton H and Kapton F appear as particularly advantageous insulating materials. In the case of Kapton F, a thermoplastic coating is provided to allow the tape to be sealed. The field of application of cables with varnished or non-varnished Kapton polyimide insulation is growing rapidly and great changes are being made therein due to their general efficiency which remains unrivalled to-day.
One of the features which is most detrimental to the development of such cables is their cost which is a function of the price of the raw materials and of the technique used in manufacturing these cables.
As far as concerns raw materials, the only true variable which appreciably influences the price of cables is the conductor nature. But it was well-known that only copper conductors with a metal coating which withstands high temperatures, e.g. silver or nickel, can be used with Kapton F polyimides.
Silver meets all technical requirements but its cost is very high.
Although nickel is not so expensive, its cost is still high and the main feature which makes it unsuitable for many users is that it is not solderable.
The need to use these expensive metal coatings was already apparent to manufacturers and users a long time ago for the technical reasons set forth hereinbelow.
Taking a known cable which includes a conductor with individual strands a Kapton polyimide taped insulation and an outer mechanically protective varnish, as far as concerns the main applications of these cables and in particular aeronautics, the cable insulation must be perfectly sealed and homogenous. Now, since polyimides are not thermoplastic materials, they cannot be applied by the extrusion technique which is the most used in conventional cable manufacture. Only a taping process can be used providing the different layers of tape are perfectly sealed together.
To be able to fulfill the above condition, it is necessary to choose tapes which are not fully polyimide but which are coated on one or both sides with a thin layer of a thermoplastic material which withstands high temperatures and will soften during heat treatment. This is the case with Kapton F polymide tape whose thermoplastic layer is fluoroethylenepropylene, or FEP, with a softening temperature of 275.degree. to 280.degree. C.
To make the outer surfaces of the cables smoother and to "level out" the edges or extra thickness related to taping techniques, it is current practice to coat such polyimide insulations with a varnish which withstands high temperatures, each successive layer of varnish deposited having to be cured. The varnishing temperature must generally be higher than 250.degree. C.
From the description of these manufacturing processes, it can be understood why conductors with silver or nickel coatings appear to be the only ones compatible with these processes, although the price of bare copper and of tinned copper is much more attractive.
Indeed, bare copper oxidizes during all these heat treatments and in use its surface condition becomes very suitable for any type of connection, either by soldering or by crimping, while tinned copper has a metal coating whose melting point (232.degree. C.) is appreciably lower than the 275.degree. C. required to seal the Kapton F polyimide tapes together and in such conditions it is, in principle, impossible to seal these tapes without making the conductors strands blocked together. Now this cannot be admitted since it is equivalent to using a conductor which has a single strand instead of a conductor with individual strands. The disadvantage of this is that a solid conductor cannot withstand the mechanical stresses which are generated in particular in an aircraft where resistance to vibrations and to bending stresses are vital requirements for cables. Only stranded conductors will meet all these requirements. The technical difficulty in manufacturing such cables is how to conciliate two requirements of supply contracts for these cables, which are:
clause 1: the tinned copper strands must not block together during the various cable manufacturing operations, so as to maintain conductor flexibility.
clause 2: the tinned copper must keep its essential intrinsic characteristics, namely:
resistance to corrosion in service, and PA1 easy connection by any soldering or crimping techniques.
To meet the requirements of both these clauses simultaneously, a known process is used at present. However, the present process is far from being sufficiently reliable for the field application. It consists in starting with a sufficient coating thickness of pure tin on the strands so as to have some left at the end of the manufacturing process, this pure tin coating being 0.7 to 1.5 microns thick.
The highest tin/copper diffusion occurs when the Kapton F polyimide tape is sealed. At this first heat treatment, (275.degree. to 280.degree. C.) the thickness of the pure tin layer is reduced by about half. Therefore, it might be considered to only partially seal the tapes i.e. only the outer tape would be completely sealed while the inner tape would be partly sealed The above process theoretically has the following advantages:
due to the thermal inertia of the insulants, there is is insufficient time to heat the conductor to a temperature exceeding. 232.degree. C.; and
the lack or imperfection of the inner tape layer sealing eliminates the pressure of the insulant on the conductor and reduces the danger of the strands being blocked together.
Although the above technique may theoretically be used, it entails the following latent and unavoidable drawbacks and dangers:
the sealing quality of the inner layers of polyimide tape is poor and cannot easily be completed during the subsequent manufacturing operations (varnishing) which are normally carried out at lower temperatures than the tape sealing operation; and
partial and not easily controllable strand blocking occurs.
Indeed, the duration and the temperature of the heat treatment must be adjusted within a very narrow operation margin and without ever being able to check that the individual strands are not being blocked together in any point along the entire length of the cable. Such a process is therefore insufficient to provide cables with the required degree of reliability.
When the above known process is rejected, the above clauses 1 and 2 remain difficult to reconcile since to meet the requirement of clause 2, the tinned copper conductor strands in the finished cable-after all the heat manufacturing treatments must show a superficial layer of pure (i.e. unalloyed) tin. Now, any heat treatment tends to cause tin to diffuse in copper, the diffusion speed depending on the temperature and the duration of the heat treatment as well as on the previous heating operations to which the tin plated copper has been subjected. Starting from the core of the wires, the metallic layers formed by such a diffusion are constituted by:
______________________________________ Cu Pure copper Both these layers are Cu.sub.3 Sn An alloy rich in copper easily oxidable Cu.sub.6 Sn.sub.5 A relatively protective alloy Sn Protective pure tin ______________________________________
It should be observed that, as soon as the tinned copper wires are manufactured, a thin inter-metallic layer of tin-copper alloys, of about 0.1 micron thick, is formed.
Thermal diffusion tends to reduce the thickness of the pure tin layer possibly until it disappears completely, giving place to a Cu.sub.6 Sn.sub.5 alloy which also--in extreme cases--may disappear, leaving only the following configuration:
Cu
Cu.sub.3 Sn
Such a result is disastrous since Cu.sub.3 Sn is easily oxidised, is not ductile and leads to textures which become fragile and brittle. Therefore, to obviate this defect, it is necessary to manufacture these cables with tinned copper strands having a minimum thickness of pure tin which is however sufficiently thick for a superficial layer of pure tin to remain on the finished cable.
In such conditions, it does not appear possible to also satisfy clause 1 except by using the previously described known process, since when the insulation tape is applied, a radial pressure is put on the conductor which is brought under pressure to a higher temperature than the melting point of tin when the layers of tape are sealed and when varnish is applied.
The present invention aims to provide a method of manufacturing a flexible electric cable with a tinned stranded conductor and insulation applied at a high temperature by which method well insulated flexible cables, which lead to easy and reliable connections, can be produced simply and safely.